PSA (Pisum sativum agglutinin) is a dimeric metalloprotein isolated from garden peas that binds saccharides and polysaccharides processing non-reducing terminal - methyl mannoside or Beta-D-glucopyranosides. The structure of this 49,000 dalton lectin has been determined to 6 Angroms resolution by protein cyrstallographic methods. The purpose of this project is to extend the resoulution of this structure to 3.0 Angstroms and then 1.7 Angstroms resolution in order to compare the three-dimensional structure with the known structure of Con A (Concanavalia ensiformis agglutinin) tetramer. This study is expected to determine th saccharide binding site by substitution of iodinated sugar in the native crystal and to compare and contrast this sie and the Ca++ and Mn++ sites with the corresponding sites in Con A. The primary sequence of PSA indicates it is related to Con A by a circular permutation of homologouos sequences. This structural study would be the first to relate two proteins that display this unusual circularly permutated sequence. The structure of PSA in addition to the Con A structure may provide a useful biochemical basis for classifying and describing the evolutionary relationships between these proteins. A comparison of the oligomeric structure of PSA and Con A will allow a detailed description of monomer-monomer and dimer-dimer contacts and how these surfaces dictate the quaternary structure.